As well known, in recent radiation therapy, advanced stereotactic radiation therapies such as three-dimensional conformal radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT) have drawn great attention (see, for example, Non-patent Document 1). In these stereotactic radiation therapies, various parameters are set using, for example, a therapy planning apparatus and thereafter irradiation with radiation is performed using an irradiator apparatus. The parameters to be set include the site and coverage of irradiation, the dose and the like of radiation, e.g., electromagnetic waves such as hard X rays and accelerated particle beams such as electron beams. By stereotactic radiation therapy, precise treatment can be accomplished, e.g., delivering a high dose of radiation only to a lesion while avoiding organs-at-risk neighboring the lesion. Hence, in the stereotactic radiation therapy, it is important to set the above various parameters to appropriate values. Therefore, a high level of mechanical accuracy of an irradiator apparatus and a high level of accuracy in the control of various filters, line width enlargcment devices and the like with which the irradiator apparatus is equipped are required.
Thus, in implementation of the above radiation therapies, it is necessary to verify various parameter values which have been set. Particularly, as to the stereoscopic dose distribution of radiation near a lesion which is to be irradiated with the radiation, many empirical data are required. Hence, conventionally, the stereoscopic dose distribution, i.e., three-dimensional dose distribution, of radiation used in therapy is measured with a polymer gel dosimeter (see, for example, Non-patent Document 1).
Meanwhile, to obtain data on the effect of radiation on the human body, it is desirable to measure the exposure dose using a dosimeter having an effective atomic number equivalent to that of living tissues constituting the human body, i.e., tissue equivalent dosimeter. As a dosimeter which is tissue equivalent to the human body, a thermoluminescent plate in the shape of a flat plate is well known (see, for example, Non-patent Document 1).
The thermoluminescent plate contains a thermoluminescent substance, i.e., thermoluminescent phosphor. The thermoluminescent phosphor comprises, for example, lithium tetraborate or the like as a base material and manganese or terbium as a luminescent center contained in the base material. By this structure, the thermoluminescent phosphor has an effective atomic number close to the effective atomic number of the human body. The thermoluminescent plate is constituted of the thermoluminescent phosphor and a heat-resistant resin which serves as a binder.
When irradiated with radiation, the thermoluminescent plate adjusted to be tissue equivalent to the human body produces effects such as photoelectric interaction, Compton effect and electron pair producing effect, and the level of the effects is the same as that in the human body. Thus, when a tissue-equivalent thermoluminescent plate is used as a dosimeter, more accurate data on the dose of radiation with which the human body is exposed can be acquired directly from measured values without making various corrections.
The thermoluminescent plate disclosed in Patent Document 1 is a plate-like product in the shape of a flat plate, as already described. The thermoluminescent plate is irradiated with radiation and then heated to thereby acquire the light intensity distribution of thermofluorescence which occurs in an exposed area of the thermoluminescent plate along a surface irradiated with the radiation. As well known, there is a certain relation between the light intensity of thermofluorescence and the radiation dose. Thus, from the light intensity distribution thus obtained, the planar exposure dose distribution (hereinafter, sometimes referred to simply as “dose distribution”), i.e., two-dimensional dose distribution, of radiation along the surface irradiated with the radiation can be acquired.